When a musical instrument, whether it is bowed or plucked, with a sound box or a resonance chamber, is played, the strings vibrate and transmit those vibrations to the resonance chamber, inside which airflow is produced due to the pressure variation determined by the strength applied to the strings.
So, the said airflow emerges as sound through the inherent holes of the instrument.
The problem lies in that inside the instrument remains a great amount of residual air which cannot get out through the holes and is blocked inside the sound box, thus producing turbulences and cramming which prevents the top and bottom plates of the instrument from wholly vibrating, thus reducing the production of harmonics.
Obviously, that restriction in harmonic production worsens to a great extent the ease of performance, the sound and timbre clarity, the volume, and the uniform sound protection.